# Melting of charge order in the low-temperature state of an electronic ferroelectric

## TL;DR

This paper investigates how charge order in a ferroelectric Mott insulator melts at low temperatures, revealing a re-entrant behavior where the ordered state disappears while the system remains insulating.

## Contribution

It demonstrates the melting of charge order in a ferroelectric Mott insulator at low temperatures, a phenomenon not previously observed in such systems.

## Key findings

- Charge order melts below 15 K while maintaining the insulating gap.
- Re-entrant behavior of charge order is observed in the phase diagram.
- Material enters a dipole solid state at 30 K before melting at lower temperatures.

## Abstract

Strong electronic interactions can drive a system into a state with a symmetry breaking. Lattice frustration or competing interactions tend to prevent a symmetry breaking, leading to quantum disordered phases. In spin systems frustration can produce a spin liquid state. Frustration of a charge degree of freedom also can result in various exotic states, however, experimental data on these effects is scarce. In this work we demonstrate how a charge ordered ferroelectric looses the order on cooling to low temperatures using an example of a Mott insulator on a weakly anisotropic triangular lattice $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl. Typically, a low temperature ordered state is a ground state of a system, and the demonstrated re-entrant behavior is unique. Raman scattering spectroscopy finds that this material enters an insulating ferroelectric `dipole solid' state at $T=30~K$, but below $T=15~K$ the order melts, while preserving the insulating energy gap. The resulting phase diagram is relevant to other quantum paraelectric materials.